Obstetric Ultrasound Study
Uterine Morphology in Pregnancy
Normal Morphology, Myometrium, Cervix, Placenta & Cesarean Scar Assessment
Uterine Morphology in Pregnancy refers to the systematic sonographic assessment of the uterus during pregnancy, including its size, shape, contour, myometrium, endometrium/gestational sac, cervix, uterine cavity, placental implantation site, and lower uterine segment. As pregnancy progresses, the uterus undergoes marked enlargement and structural changes to accommodate the growing fetus and placenta. Ultrasound evaluation helps assess normal pregnancy-related changes and identify abnormalities such as fibroids, adenomyosis, uterine anomalies, scar-related changes, cervical abnormalities, abnormal placentation, and lower-segment pathology.
Uterine Size & Shape → During pregnancy, the uterus progressively enlarges from a small pelvic organ into an abdominal organ. Its shape changes from pear-shaped in early pregnancy to a more globular and subsequently ovoid configuration as gestation advances. The uterine contour should remain smooth and well defined.
What to assess:
• Overall uterine size and enlargement appropriate for gestational age.
• Uterine contour — smooth and regular or irregular.
• Symmetry of the uterine walls.
• Presence of focal bulging or masses.
• Relationship of the uterus to the fetal presenting part and placenta.
• Any distortion of the uterine cavity.
A uterus that is disproportionately large for gestational age may be associated with multiple pregnancy, polyhydramnios, uterine fibroids, molar pregnancy, or incorrect dating, whereas a small uterus may require correlation with gestational age and fetal growth.
Myometrium → The myometrium normally becomes progressively thinner and more distensible as pregnancy advances. Sonographic assessment should include the uniformity, echotexture, thickness, and focal lesions of the myometrium.
Normal appearance:
The myometrium is generally homogeneous in early pregnancy and becomes increasingly heterogeneous later in pregnancy because of myometrial stretching, vascularity, and uterine contractions.
Important abnormalities:
• Leiomyoma (fibroid): well-defined hypoechoic or heterogeneous mass arising from the myometrium.
• Adenomyosis: heterogeneous myometrium, myometrial cysts, fan-shaped shadowing, or poorly defined endomyometrial junction.
• Uterine anomaly: abnormal uterine contour or cavity configuration.
• Scar-related thinning: particularly important in patients with previous cesarean delivery.
• Focal myometrial tenderness or hypervascularity: should be interpreted with clinical findings.
Uterine Cavity → The uterine cavity should be evaluated for the location and development of the gestational sac, embryo/fetus, placenta, membranes, and amniotic cavity.
In early pregnancy, ultrasound evaluates the gestational sac, yolk sac, embryo, crown-rump length, and cardiac activity. In later pregnancy, attention shifts toward the fetal position, placental location, membranes, amniotic fluid, and relationship of the placenta to the cervix.
The cavity should appear appropriately distended by the pregnancy without abnormal intracavitary masses, collections, or significant distortion.
Uterine Contour → The external uterine contour should be assessed in multiple planes. A smooth contour is expected. Focal or generalized distortion may result from fibroids, congenital uterine anomalies, previous surgery, adenomyosis, or abnormal placental implantation.
Evaluate for:
1. Regularity of the anterior and posterior uterine walls.
2. Fundal contour.
3. Lateral uterine borders.
4. Any focal bulge or indentation.
5. Distortion caused by fibroids or previous surgical scars.
6. Relationship of the uterine contour to the placenta.
Placental Implantation Site → Placental location is an essential component of uterine morphology assessment. The placenta should be localized with respect to the fundus, anterior wall, posterior wall, lateral wall, and internal cervical os.
Assess:
• Placental position and extent.
• Distance between the placental edge and internal cervical os when clinically indicated.
• Placental thickness and echotexture.
• Focal placental masses or abnormal collections.
• Relationship between the placenta and previous uterine scar.
• Sonographic signs suspicious for placenta accreta spectrum (PAS) when risk factors are present.
In patients with a previous cesarean section, particular attention should be paid to the anterior lower uterine segment and placental implantation over or near the scar.
Lower Uterine Segment (LUS) → The lower uterine segment becomes increasingly important in the second and third trimesters, especially in patients with a history of cesarean delivery.
Ultrasound assessment includes the myometrial appearance, scar region, relationship of the placenta to the scar, and any abnormal vascularity. Marked thinning should be interpreted in conjunction with gestational age, previous surgical history, and clinical findings.
Important observations:
• Integrity and appearance of previous cesarean scar.
• Focal or generalized LUS thinning.
• Placental position relative to the scar.
• Abnormal bridging vessels or hypervascularity.
• Bulging or focal discontinuity of the myometrium.
• Features suspicious for scar pregnancy or placenta accreta spectrum when clinically relevant. Reference Values for Anterior Myometrial Thickness (AMT) and Residual Myometrial Thickness (RMT)
Note: AMT/RMT values vary with gestational age, measurement technique, bladder filling, uterine position and previous cesarean-scar morphology. RMT normally decreases as pregnancy progresses. Published studies report first-trimester RMT around 4.7–5.2 mm, with progressive thinning later in pregnancy. A very thin RMT (particularly around ≤2–2.2 mm) is considered concerning in the context of a cesarean-scar niche, but should not be interpreted in isolation.
If RMT (residual myometrial thickness) is <3 mm over a previous cesarean scar, it is generally described as thin residual myometrium.
Other abnormalities in Uterine scar:
1. Cesarean scar niche / isthmocele
2. Scar dehiscence
3. Uterine scar rupture
4. Markedly thin residual myometrial thickness (RMT)
5. Scar diverticulum
6. Scar fibrosis / irregular scar
7. Scar hematoma
8. Cesarean scar ectopic pregnancy (CSP)
9. Placenta previa/accreta spectrum involving the scar
10. Scar endometriosis
Cervix → The cervix should be assessed for length, internal os, external os, cervical canal, and relationship to the presenting part. Transvaginal ultrasound provides the most reliable assessment of cervical length when indicated.
Normal cervical assessment:
The cervix generally appears closed with a preserved cervical canal and adequate cervical length. The internal os should be evaluated for funneling or opening when clinically indicated.
Abnormal findings may include short cervix, funneling, cervical dilatation, cervical masses, or cervical incompetence.
Uterine Fibroids in Pregnancy → Fibroids are common benign uterine smooth-muscle tumors and may be detected incidentally during pregnancy. Ultrasound should document their number, location, size, morphology, and relationship to the placenta and uterine cavity.
Describe:
• Intramural, submucosal, or subserosal location.
• Maximum dimensions in three planes.
• Number of fibroids.
• Degenerative changes.
• Distortion of the uterine cavity.
• Relationship to placenta and cervix.
Fibroids may enlarge, remain stable, or undergo degenerative changes during pregnancy. Their clinical significance depends on size, location, number, and relationship to the placenta or cervix.
Uterine Contractions → Transient focal myometrial thickening or distortion may represent a physiological uterine contraction. A contraction can temporarily alter uterine contour and may mimic a mass or myometrial abnormality.
When an apparent focal lesion is identified, reassessment after a short interval can help distinguish a transient contraction from a persistent structural lesion such as a fibroid.
Sonographic Documentation → A complete uterine morphology assessment should document the following parameters according to gestational age and clinical indication:
Practical scanning technique:
1. Examine the uterus in sagittal and transverse planes.
2. Assess the overall uterine contour and myometrial echotexture.
3. Identify the placenta and document its location.
4. Assess the lower uterine segment, particularly after previous cesarean delivery.
5. Evaluate the cervix when clinically indicated, preferably by transvaginal ultrasound for cervical-length assessment.
6. Document any fibroid, adenomyosis, scar abnormality, or focal myometrial lesion.
7. Use color Doppler when abnormal vascularity or placenta accreta spectrum is suspected.
8. Correlate uterine morphology with gestational age, obstetric history, symptoms, fetal growth, and placental findings.
Key Interpretation → A normal pregnancy-related uterus demonstrates progressive enlargement, smooth contour, appropriate myometrial remodeling, normal placental implantation, and an appropriately closed cervix for the gestational age. Any structural abnormality should be described according to its location, size, morphology, vascularity, and relationship to the placenta, cervix, uterine cavity, or previous surgical scar.
Uterine Size & Shape → During pregnancy, the uterus progressively enlarges from a small pelvic organ into an abdominal organ. Its shape changes from pear-shaped in early pregnancy to a more globular and subsequently ovoid configuration as gestation advances. The uterine contour should remain smooth and well defined.
What to assess:
• Overall uterine size and enlargement appropriate for gestational age.
• Uterine contour — smooth and regular or irregular.
• Symmetry of the uterine walls.
• Presence of focal bulging or masses.
• Relationship of the uterus to the fetal presenting part and placenta.
• Any distortion of the uterine cavity.
A uterus that is disproportionately large for gestational age may be associated with multiple pregnancy, polyhydramnios, uterine fibroids, molar pregnancy, or incorrect dating, whereas a small uterus may require correlation with gestational age and fetal growth.
Myometrium → The myometrium normally becomes progressively thinner and more distensible as pregnancy advances. Sonographic assessment should include the uniformity, echotexture, thickness, and focal lesions of the myometrium.
Normal appearance:
The myometrium is generally homogeneous in early pregnancy and becomes increasingly heterogeneous later in pregnancy because of myometrial stretching, vascularity, and uterine contractions.
Important abnormalities:
• Leiomyoma (fibroid): well-defined hypoechoic or heterogeneous mass arising from the myometrium.
• Adenomyosis: heterogeneous myometrium, myometrial cysts, fan-shaped shadowing, or poorly defined endomyometrial junction.
• Uterine anomaly: abnormal uterine contour or cavity configuration.
• Scar-related thinning: particularly important in patients with previous cesarean delivery.
• Focal myometrial tenderness or hypervascularity: should be interpreted with clinical findings.
Uterine Cavity → The uterine cavity should be evaluated for the location and development of the gestational sac, embryo/fetus, placenta, membranes, and amniotic cavity.
In early pregnancy, ultrasound evaluates the gestational sac, yolk sac, embryo, crown-rump length, and cardiac activity. In later pregnancy, attention shifts toward the fetal position, placental location, membranes, amniotic fluid, and relationship of the placenta to the cervix.
The cavity should appear appropriately distended by the pregnancy without abnormal intracavitary masses, collections, or significant distortion.
Uterine Contour → The external uterine contour should be assessed in multiple planes. A smooth contour is expected. Focal or generalized distortion may result from fibroids, congenital uterine anomalies, previous surgery, adenomyosis, or abnormal placental implantation.
Evaluate for:
1. Regularity of the anterior and posterior uterine walls.
2. Fundal contour.
3. Lateral uterine borders.
4. Any focal bulge or indentation.
5. Distortion caused by fibroids or previous surgical scars.
6. Relationship of the uterine contour to the placenta.
Placental Implantation Site → Placental location is an essential component of uterine morphology assessment. The placenta should be localized with respect to the fundus, anterior wall, posterior wall, lateral wall, and internal cervical os.
Assess:
• Placental position and extent.
• Distance between the placental edge and internal cervical os when clinically indicated.
• Placental thickness and echotexture.
• Focal placental masses or abnormal collections.
• Relationship between the placenta and previous uterine scar.
• Sonographic signs suspicious for placenta accreta spectrum (PAS) when risk factors are present.
In patients with a previous cesarean section, particular attention should be paid to the anterior lower uterine segment and placental implantation over or near the scar.
Lower Uterine Segment (LUS) → The lower uterine segment becomes increasingly important in the second and third trimesters, especially in patients with a history of cesarean delivery.
Ultrasound assessment includes the myometrial appearance, scar region, relationship of the placenta to the scar, and any abnormal vascularity. Marked thinning should be interpreted in conjunction with gestational age, previous surgical history, and clinical findings.
Important observations:
• Integrity and appearance of previous cesarean scar.
• Focal or generalized LUS thinning.
• Placental position relative to the scar.
• Abnormal bridging vessels or hypervascularity.
• Bulging or focal discontinuity of the myometrium.
• Features suspicious for scar pregnancy or placenta accreta spectrum when clinically relevant. Reference Values for Anterior Myometrial Thickness (AMT) and Residual Myometrial Thickness (RMT)
| GA (wk) | AMT (mm) | RMT (mm) | ||||
| Lower | Typical | Upper | Lower | Typical | Upper | |
| 11–13 | 10 | 12–13 | 15 | 4 | 5–6 | 8 |
| 14–18 | 8 | 10–11 | 14 | 3.5 | 4.5–5 | 7 |
| 19–22 | 6 | 8–9 | 12 | 3 | 4 | 6 |
| 23–27 | 5 | 7–8 | 10 | 2.5 | 3–4 | 5 |
| 28–32 | 4 | 6–7 | 9 | 2 | 2.5–3.5 | 5 |
| 33–36 | 3.5 | 5–6 | 8 | 1.5 | 2–3 | 4 |
| 37–40 | 3 | 4–5 | 7 | 1.5 | 2–2.5 | 4 |
Note: AMT/RMT values vary with gestational age, measurement technique, bladder filling, uterine position and previous cesarean-scar morphology. RMT normally decreases as pregnancy progresses. Published studies report first-trimester RMT around 4.7–5.2 mm, with progressive thinning later in pregnancy. A very thin RMT (particularly around ≤2–2.2 mm) is considered concerning in the context of a cesarean-scar niche, but should not be interpreted in isolation.
If RMT (residual myometrial thickness) is <3 mm over a previous cesarean scar, it is generally described as thin residual myometrium.
- 2–3 mm: thin/borderline — interpret with gestational age, technique, and symptoms.
- <2 mm: markedly thin — more concerning for scar dehiscence / uterine scar weakness.
- Absent myometrial layer: highly concerning for scar dehiscence or rupture, depending on the ultrasound appearance.
Other abnormalities in Uterine scar:
1. Cesarean scar niche / isthmocele
2. Scar dehiscence
3. Uterine scar rupture
4. Markedly thin residual myometrial thickness (RMT)
5. Scar diverticulum
6. Scar fibrosis / irregular scar
7. Scar hematoma
8. Cesarean scar ectopic pregnancy (CSP)
9. Placenta previa/accreta spectrum involving the scar
10. Scar endometriosis
Cervix → The cervix should be assessed for length, internal os, external os, cervical canal, and relationship to the presenting part. Transvaginal ultrasound provides the most reliable assessment of cervical length when indicated.
Normal cervical assessment:
The cervix generally appears closed with a preserved cervical canal and adequate cervical length. The internal os should be evaluated for funneling or opening when clinically indicated.
Abnormal findings may include short cervix, funneling, cervical dilatation, cervical masses, or cervical incompetence.
Uterine Fibroids in Pregnancy → Fibroids are common benign uterine smooth-muscle tumors and may be detected incidentally during pregnancy. Ultrasound should document their number, location, size, morphology, and relationship to the placenta and uterine cavity.
Describe:
• Intramural, submucosal, or subserosal location.
• Maximum dimensions in three planes.
• Number of fibroids.
• Degenerative changes.
• Distortion of the uterine cavity.
• Relationship to placenta and cervix.
Fibroids may enlarge, remain stable, or undergo degenerative changes during pregnancy. Their clinical significance depends on size, location, number, and relationship to the placenta or cervix.
Uterine Contractions → Transient focal myometrial thickening or distortion may represent a physiological uterine contraction. A contraction can temporarily alter uterine contour and may mimic a mass or myometrial abnormality.
When an apparent focal lesion is identified, reassessment after a short interval can help distinguish a transient contraction from a persistent structural lesion such as a fibroid.
Sonographic Documentation → A complete uterine morphology assessment should document the following parameters according to gestational age and clinical indication:
| Parameter | Sonographic Assessment |
|---|---|
| Uterine size | Appropriate for gestational age; overall enlargement |
| Uterine contour | Smooth, regular or distorted |
| Myometrium | Thickness, echotexture, fibroids, adenomyosis, scars |
| Uterine cavity | Gestation, fetus, membranes and cavity distortion |
| Placenta | Location, morphology and relationship to internal os/scar |
| Lower uterine segment | Scar region, myometrial appearance and placental relationship |
| Cervix | Length, canal, internal os and funneling when indicated |
| Adnexa | Ovaries/adnexal masses when clinically indicated |
Practical scanning technique:
1. Examine the uterus in sagittal and transverse planes.
2. Assess the overall uterine contour and myometrial echotexture.
3. Identify the placenta and document its location.
4. Assess the lower uterine segment, particularly after previous cesarean delivery.
5. Evaluate the cervix when clinically indicated, preferably by transvaginal ultrasound for cervical-length assessment.
6. Document any fibroid, adenomyosis, scar abnormality, or focal myometrial lesion.
7. Use color Doppler when abnormal vascularity or placenta accreta spectrum is suspected.
8. Correlate uterine morphology with gestational age, obstetric history, symptoms, fetal growth, and placental findings.
Key Interpretation → A normal pregnancy-related uterus demonstrates progressive enlargement, smooth contour, appropriate myometrial remodeling, normal placental implantation, and an appropriately closed cervix for the gestational age. Any structural abnormality should be described according to its location, size, morphology, vascularity, and relationship to the placenta, cervix, uterine cavity, or previous surgical scar.
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